1. Field of the Invention
The invention relates to a liquid crystal display device having an area through which a light is allowed to pass for forming images on a screen, and an area in which a light is reflected for forming images on a screen.
2. Description of the Related Art
A liquid crystal display device has advantages that it is thin, and it consumes small power. Due to the advantages, a liquid crystal display device is widely used, for instance, for a monitoring screen in various displays.
Unlike a cathode ray tube (CRT) and an electroluminescence (EL) display device, a liquid crystal display device does not have a function of emitting a light. Hence, a liquid crystal display device is necessary to have a light source separately from a monitoring screen. In dependence on a type of a light source, a liquid crystal display device is grouped into a light transmission type liquid crystal display device and a light reflection type liquid crystal display device.
A light transmission type liquid crystal display device includes a light source at a rear, and display images by switching transmission and interruption of a light (which is called “backlight”) emitted from the light source.
In a light transmission type liquid crystal display device, power consumed for emitting a backlight is 50% or greater in total power consumption. That is, a light source for emitting a backlight causes much power consumption in a light transmission type liquid crystal display device.
In order to solve such a disadvantage of a light transmission type liquid crystal display device as mentioned above, there has been suggested a light reflection type liquid crystal display device.
A light reflection type liquid crystal display device includes a light-reflection plate at which surrounding lights are reflected. Images are displayed on a screen by switching transmission and interruption of lights having been reflected from the light-reflection plate. Unlike a light transmission type liquid crystal display device, a light reflection type liquid crystal display device is not necessary to include a light source, and hence, can reduce power consumption in comparison with a light transmission type liquid crystal display device.
For instance, a handy communication device such as a cellular phone is frequently used outside, and hence, can use surrounding lights as reflected lights. For that reason, a light reflection type liquid crystal display device is suitable to a monitoring screen of a handy communication device.
However, a light reflection type liquid crystal display device which uses surrounding lights as reflected lights is accompanied with a problem that if surrounding lights are not bright, a user cannot clearly look at a screen.
On the other hand, contrary to a light reflection type liquid crystal display device, a light transmission type liquid crystal display device is accompanied with a problem that if surrounding lights are extremely bright, images displayed on a screen look dark in comparison with the surrounding lights.
In order to solve those problems, there has been suggested a light reflection and transmission type liquid crystal display device which accomplish both displaying images by partially transmitting a light and displaying images by partially reflecting a light, in a single liquid crystal display panel.
As an example, a light reflection and transmission type liquid crystal display device suggested in Japanese Patent No. 2955277 (B2) (Japanese Unexamined Patent Publication No. 11-101992 (A)) is illustrated in FIG. 1.
The illustrated liquid crystal display device is comprised of an active matrix substrate 100, an opposing substrate 110 facing the active matrix substrate 100, and a liquid crystal layer 120 sandwiched between the active matrix substrate 100 and the opposing substrate 110.
The active matrix substrate 100 is comprised of a first transparent substrate 101, a retardation plate 102 formed on the first transparent substrate 101 at the opposite side of the liquid crystal layer 120, and a polarizer 103 formed on the retardation plate 102.
A backlight source 104 is arranged below the polarizer 103.
The opposing substrate 110 is comprised of a second transparent substrate 111, a retardation plate 112 formed on the second transparent substrate 111, and a polarizer 113 formed on the retardation plate 112.
The first transparent substrate 101 has a first area A in which a light emitted from the backlight source 104 transmits through the active matrix substrate 100, the liquid crystal layer 120 and the opposing substrate 110, and a second area B in which an incoming light is reflected.
An electrically conductive transparent film 105 is formed on the first transparent substrate 101 in the first area A.
An electrically insulating film 106 having raised and recessed portions, and a light-reflection plate 107 covering the electrically insulating film 106 therewith are formed on the first transparent substrate 101 in the second area B.
A light 130 having been emitted from the backlight source 104 passes through the electrically conductive transparent film 105 for forming certain images on a liquid crystal display panel. A light 140 externally entering the liquid crystal display device is reflected at the light-reflection plate 107 for forming certain images on a liquid crystal display panel.
The retardation plates 102 and 112 provide retardation of a quarter length (λ/4) to such a transmission light and a reflected light. A transmission light and a reflected light are converted into a linearly polarized light from a circularly polarized light or vice versa, when they pass through the polarizers 103 and 113.
As illustrated in FIG. 1, the electrically insulating film 106 is not formed below the electrically conductive transparent film 105. Hence, it would be possible to equalize an optical length of a light in the liquid crystal layer 120 which light passes through the liquid crystal layer 120 in reciprocation in the second area B, and an optical length of a light in the liquid crystal layer 120 which light passes through the liquid crystal layer 120 in the first area A, to each other by designing a cell gap in the first area A, that is, a thickness Df of the liquid crystal layer 120 to be greater than a cell gap Dr in the second area B. As a result, it would be possible to control optical characteristics of lights passing through the liquid crystal layer 120 in the first area A and in the second area B such that the optical characteristics vary in the same manner.
However, the conventional liquid crystal display device illustrated in FIG. 1 is accompanied with a problem that if characteristics of displaying images through reflected lights are optimized, a luminance would be lowered, because the cell gap Df in the first area A is greater than the cell gap Dr in the second area B. This is because a twisting angle of liquid crystal is equal to about 72 degrees, if the above-mentioned characteristics are optimized, and an intensity of a transmission light is lowered at a twisting angle of about 72 degrees under the condition of Df<Dr.
Japanese Unexamined Patent Publication No. 2000-258769 (A) has suggested a liquid crystal display device including a first polarizer, a second polarizer, a liquid crystal device sandwiched between the first and second polarizers, a retardation plate arranged between the second polarizer and the liquid crystal device, and a light source arranged on the second polarizer at the opposite side of the retardation plate. The liquid crystal device is a transmission and reflection type device, and is comprised of a first light-permeable substrate arranged on the first polarizer, a second substrate arranged on the retardation plate and having a light-reflection area and a light-transmission area, and a liquid crystal layer which is arranged between the first and second substrates, is horizontally aligned, and has positive dielectric anisotropy. The retardation plate compensates for a viewing angle, and has an index ellipsoid having a main index of refraction inclining relative to a normal line of the second substrate.
However, the above-mentioned problems remain unsolved even in the liquid crystal display device suggested in the Publication.